1) Field of the Invention
The field of the present invention relates to sound reproduction and, more specifically, to a speaker configuration and related sound processing for use in an automobile or vehicular sound system.
2) Background
Audio systems are commonplace in automobiles and certain other vehicles. Such systems generally utilize program sources ranging from simple radios to relatively elaborate stereo or multi-channel systems with CD and cassette players together with multiple equalizers, pre-amplifiers, power amplifiers etc.
While there is a great variety in the configuration and components of conventional automotive audio systems, most of them suffer to varying degrees from a number of persistent problems in providing the highest sound quality. These problems partially result from the unique sound environment of the automobile when compared with a good listening room. Among the disadvantages are:                Much smaller internal volume resulting in a reduced reverberation time and lower modal density at low frequencies resulting in a lack of ambience and an uneven bass response.        The proximity of highly reflective surfaces (such as the windows) to highly absorptive areas such as the upholstery or the occupants clothing leads to a great variability with frequency and head position of the direct to indirect sound arriving at the listener. Consequently even small changes in head or seating position can cause significant and undesirable changes in the timbral quality of the music.        The listening positions are necessarily restricted to the seating positions provided (usually 4 or 5) and all of these are very asymmetrically placed with respect to the speaker positions. Space is always at a premium within a car interior and as a result the speakers are often placed in physically convenient positions, that are nevertheless very poor from an acoustic point of view, such as the foot wells and the bottom of the front and rear side doors. As a result the listener's head is always much closer to either the left or right speaker leading directly large inter-channel time differences and different sound levels due to the 1/r law.        
Additionally, the angles between the axes from the listeners ears to the axes of symmetry of the left and right speakers is quite different for each occupant. The perceived spectral balance is different for each channel due to the directional characteristics of the drive units. Masking of one or more speakers by the occupants clothes or legs can often result in the attenuation of the mid- and high-frequencies by as much as 10 dB.
All of the above adversely impact the ability to produce high quality stereo reproduction, which ideally has the following attributes:
                A believable and stable image or soundstage resulting from the listener being nearly equidistant from the speakers reproducing the left and right channels and a sufficiently high ratio of direct-to-indirect sound at the listener's ears.        A smooth timbral balance at all the listening positions.        A sense of ambience resulting from a uniform soundfield.        
Some features are provided in automobile audio systems which can partially mitigate the aforementioned problems. For example, an occupant can manually adjust the sound balance to increase the proportional volume to the left or right speakers. Some automobile audio systems have a “driver mode” button which makes the sound optimal for the driver. However, because different listening axes exist for left and right occupants, an adjustment to the balance that satisfies the occupant (e.g., driver) on one side of the automobile will usually make the sound worse for the occupant seated on the other side of the automobile. Moreover, balance adjustment requires manual adjustment by one of the occupants, and it is generally desirable in an automobile to minimize user intervention.
Another modification made to some automobile audio systems is to provide a center speaker, which reduces the image instability that occurs when the listener is closer to either the left or right speaker when both are reproducing the same mono signal, with the intention of producing a central sound image. Other potential approaches which might be taken in an attempt to mitigate the foregoing automotive sound problems include adding more speakers in a greater variety of positions (e.g., at the seat tops). While such techniques can sometimes provide a more pleasing effect, they cannot provide stable imaging as the problems associated with asymmetry described above still remain. The considerable additional cost of such design approaches is usually undesirable in the highly cost sensitive and competitive automotive industry. Moreover, as previously noted, space is usually at a premium in the automobile interior, and optimal speaker positions are limited.
Accordingly, it would be advantageous to provide an improved automotive sound system which overcomes one or more of the foregoing problems or shortcomings, and which can provide improved sound quality while minimizing any increase in cost of the audio system.